The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to, a method and apparatus to acquire MR images with improved image signal and contrast using an interleaved, notched RF saturation pulse as applied to MRI perfusion imaging.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Myocardial perfusion imaging is the detection of a contrast agent as it passes through the muscle tissue in the heart to non-invasively study blood flow in the micro-circulation of the heart. Typically, perfusion imaging consists of using an injected contrast agent (bolus) with rapid imaging during the first pass of the bolus using carefully optimized pulse sequence parameters. Quantification of blood flow from these images is accomplished with a region-of-interest based signal, time-intensity curve analysis. To avoid cardiac motion artifacts, the perfusion images are typically acquired with ECG gating to synchronize the repeated acquisition of images at different spatial locations, each to the same relative point in the cardiac cycle. In the past, the period of image acquisition was typically several minutes long, causing the images to suffer from significant respiratory motion artifacts. Such artifacts would require the manual registration and analysis of the perfusion images which is cumbersome and time-consuming because the user must carefully arrange each image to compensate for the respiratory motion before proceeding to a region-of-interest, time-intensity analysis. Furthermore, the passage of the contrast agent takes place over a temporal span of several seconds. By averaging over several seconds or minutes, the effectiveness of measuring any change in perfusion is severely compromised.
The goal of myocardial perfusion imaging is to detect and characterize the abnormal distribution of myocardial blood flow. The ability to extract quantitative perfusion indices such as time-to-peak, contrast enhancement ratio, and the slope from the first-pass contrast-enhanced MR images requires a generation of myocardial and blood-pool time-intensity curves for desired regions-of-interest. The computation of these curves is complicated when patients do not suspend respiration adequately, which then results in an image misregistration over time. Mis-registration artifacts occur frequently due to the fact that the breath-hold duration required to capture first-pass kinetics is typically 20-30 seconds. An accurate spatial alignment of images over a period of time is necessary for creating representative and accurate time-intensity curves for a given region of the myocardium.
The imaging of blood perfusion in tissue is closely related to the imaging of blood flow in vascular structures, such as in MR angiography. As with MR angiography, MR perfusion imaging is performed by injecting the bolus of an MR active contrast agent into the patient during an imaging session. These agents can either decrease the T1 of blood to enhance the detected MR signal, or decrease the T2 of blood to attenuate the detected MR signal. As the bolus passes through the body, the enhanced or attenuated signal increases or decreases the signal intensity observed in perfused tissue, but not in non-perfused tissue. The degree of signal change in the observed tissue can be used to determine the degree of tissue perfusion. Since perfusion measurements are based on the strength of the MR signals acquired during the scan, it is important that the MR signal strength be made insensitive to other measured variables. One such variable is the magnitude of the longitudinal magnetization M2, which is tipped into the transverse plane by the RF excitation pulse in the MR pulse sequence. After each such excitation, the longitudinal magnetization is reduced and then recovers magnitude at a rate determined by the T1 constant of the particular spins being imaged. If another pulse sequence is performed before the longitudinal magnetization has recovered, the magnitude of the acquired MR signal will be less than the signal produced by a pulse sequence which is delayed long enough to allow full recovery of the longitudinal magnetization. It is therefore important in perfusion imaging that the longitudinal magnetization variable be maintained at a constant level throughout the scan.
When cardiac gating is used to control the acquisition of MR perfusion image data, the time interval between acquisitions can vary considerably with a consequent variation in the longitudinal magnetization. This is particularly true if the subject has an irregular heartbeat (arrhythmia) or other variations in the heart rate. One solution to this problem is to apply an RF saturation pulse to the subject just prior to each image acquisition pulse sequence, or imaging pulse sequence segment, for each slice, and allow a fixed recovery time (TI) to occur before performing the pulse sequence. Unfortunately, unless the recovery time TI is fairly lengthy, the resulting MR signals will not have significant contrast between tissues of differing T1 relaxation times, in addition to the MR signal being small, with a consequent reduction in the acquired MR signals, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR). However, lengthening the recovery period TI lengthens the time required to perform each pulse sequence segment and reduces the number of slice locations that can be acquired during a cardiac R-R interval. Therefore, there is a direct tradeoff between image quality and the number of locations that can be acquired in a single breath-hold scan.
One particular method includes preparing a given slice with a non-selective 45-60xc2x0 pulse which is allowed to recover for a time TI, then acquire data with an echo planar imaging (EPI) readout. However, with this implementation, the 45-60xc2x0 preparation pulse provides only weak T1 weighting and can allow signal variation due to variations in the patient""s cardiac interval arrhythmias. Further, the short TI (approximately 10 ms) does not allow development of sufficient image SNR or contrast. In other words, the low preparation flip angle of 45-60xc2x0 does not allow for the development of adequate contrast and such a low excitation flip angle does not provide adequate SNR. Replacing the short TI partial-saturation preparation sequence with a long TI saturation-recovery sequence would address this one problem, but causes another by reducing slice coverage.
It would thus be desirable to have a means for acquiring MR perfusion images that does not affect the slice that is to be immediately acquired so as to obtain longer TI times, while maintaining the ability to acquire multiple slices per R-R interval, provide a high degree of immunity to the effects of arrhythmias and other variations in a patient""s heart rate, and offer blood pool suppression (to better delineate the myocardial tissue).
The present invention relates to a system and method of acquiring MR data using an interleaved, notched RF saturation pulse that provides adequate slice coverage, good overall SNR, and sufficient contrast between enhanced and unenhanced myocardium in myocardial perfusion imaging, overcoming the aforementioned problems.
Rather than using a non-selective partial-saturation pulse, the present invention uses a volume-selective RF saturation pulse with a stop-band, or xe2x80x9cnotched,xe2x80x9d sliced profile. The notch is designed to coincide with the slice location that will be imaged by a following data acquisition. Preferably, the width of the notch is user selectable and slightly greater than the imaged slice thickness (to account for cardiac or respiratory motion during the TI time). In MR perfusion studies, the notched pulse saturates all the spins outside of the notched stop-band. This results in the saturation of the blood in the ventricular chambers and provides a high degree of immunity to the effects caused by arrhythmias or other variations in the patient""s heart rate. The notched saturation pulse does not affect the slice to be immediately acquired after the transmission of the pulse so longer TI times are attainable while maintaining the ability to obtain at least 3-4 slices per R-R interval.
In accordance with one aspect of the invention, a method of acquiring MR data includes the steps of selecting a volume of slice locations in which MR data is to be acquired, transmitting a notched RF saturation pulse within the selected volume of slice locations, wherein the notched RF saturation pulse has a stop-band situated between a pair of pass-bands, and then acquiring MR data for the slice location that was in the stop-band of the notched RF saturation pulse previously transmitted.
In accordance with another aspect of the invention, a method of acquiring MR data with longer relaxation time (TI) includes the steps of defining a number of slices in a volume of interest for which acquisition of MR data is desired, then transmitting a notched RF saturation pulse designed to saturate a next slice and all surrounding tissue in the volume of interest except a current slice, and acquiring MR data from the current slice. The process repeats the transmission and acquisition steps for each successive slice in the volume of interest wherein the next slice becomes the current slice and another slice becomes the next slice.
In accordance with yet another aspect of the invention, a computer system is disclosed for use with an MRI apparatus having a computer programmed with a computer readable storage medium having thereon a computer program programmed to select a volume of slice locations in which MR data is to be acquired, transmit a notched RF saturation pulse within the selected volume of slice locations, the notched RF saturation pulse having a stop-band between a pair of pass-bands, and acquire MR data in the stop-band of the notched RF saturation pulse.